EFFECT OF SOIL ORGANIC CARBON CONTENT ON RIPARIAN NITRATE ATTENUATION DURING STREAM STAGE FLUCTUATIONS

Riparian zones occur at the interface between surface water and groundwater. These zones are critical locations for the occurrence of biogeochemical processes such as denitrification. Denitrification is an important natural process where nitrate pollution resulting from anthropogenic inputs to streams and groundwater is removed. Stream stage fluctuations leading to increased bank storage have a strong effect on the removal of river-borne nitrate through facultative anaerobic microbial denitrification, referred to as “hot moments.” Since the availability of oxygen and organic carbon are considered to exert the greatest control, denitrification can be significant when the water table rises to reach the organic carbon-rich topsoils. Previous studies on denitrification during hot moments have focused on hydrologic controls as well as stream dissolved organic carbon input. This study expands upon this knowledge by examining the relationship between the depth profile of soil organic carbon and the rate of denitrification in the riparian zone during hot moments using a two-dimensional cross-sectional groundwater flow (MODFLOW) and transport (MT3D) model. The model is applied to simulate hyporheic exchange and nitrate removal over periods of stream stage rise and fall using data from Boone Creek in Boone, North Carolina. The effect of organic carbon on denitrification can be approximated using first-order decay constants. We treat the nitrate decay constant as a proxy for soil organic carbon content and subsequently model the transport and removal of nitrate in the riparian zone. Results from this two-dimensional simulation will allow for better predictions on the occurrence and magnitude of denitrification hot moments in riparian zones as well as provide insight into the importance of topsoil in controlling the removal of nitrate from both surface water and groundwater.